Over the past thirty years, significant attention has been given to the production of ethyl alcohol, or “ethanol,” for use as an alternative fuel. Ethanol not only burns cleaner than fossil fuels, but also can be produced using corn, a renewable resource. At present, an estimated sixty-nine “dry milling” plants in the United States produce over three billion gallons of ethanol per year. Additional plants presently under construction are expected to add billions of gallons to this total in an effort to meet the current high demand.
As noted in the foregoing discussion, a popular method of producing ethanol from corn is known as “dry milling.” As is well known in the industry, the dry milling process utilizes the starch in the corn to produce the ethanol through fermentation, and creates a waste stream or byproduct termed “whole stillage” (which may be further separated into byproducts commonly referred to as “distillers wet grains” and “thin stillage”). Despite containing valuable oil, these byproducts have for the most part been treated as waste and used primarily to supplement animal feed. This feed is mostly distributed in the form of distillers dried grains with solubles, which is created by evaporating the thin stillage, recombining the resulting concentrate or syrup with the distillers wet grains, and drying the product to a moisture content of less than about 10% by weight.
Significant attention has recently been given to the use of oil, including corn oil, as an alternative fuel. This fuel oil, frequently termed “biodiesel”, is a cleaner fuel than petroleum-based diesel (less emissions), environmentally safe (spills biodegrade quickly), and can be mixed at any concentration to diesel without engine modification. The current value of corn oil as biodiesel is approximately $2.40 per gallon, or $648/ton, which is essentially double the value of the commercial feed that would normally include this oil. Although the market for the biodiesel is growing rapidly and the potential profit is significant, key limiting factors are the cost of obtaining the oil using current techniques and the resulting quality.
In this regard, efforts to recover usable oil from the byproducts of the dry milling process used to create ethanol have not been terribly successful in terms of efficiency. One proposed approach involves attempting to separate the oil from the thin stillage before the evaporation stage, such as using a centrifuge. However, spinning the thin stillage at this stage using a centrifuge creates an emulsion phase that typically requires further processing before useable oil can be recovered. Moreover, the volume of thin stillage is generally 2 to 10 times greater than the syrup, which is a considerable capital requirement to purchase the number of centrifuges required. Known techniques also lack the capability to maximize the oil recovery, which leads to decreased efficiency and concomitant lower profits. Together, these obstacles make past and current attempts to recover oil from ethanol production byproducts, such as corn thin stillage, somewhat inefficient and uneconomical.
U.S. Pat. No. 5,250,182 (the disclosure of which is incorporated herein by reference) describes the use of filters for removing substantially all solids and recovering lactic acid and glycerol from the thin stillage without the need for evaporation. Despite eliminating a step in the conventional process, the proposal results in a more complicated arrangement requiring multiple filtration steps. Wholesale elimination of the evaporator in the vast majority of existing plants is also unlikely and otherwise uneconomical. Filters, and especially the microfiltration and ultrafiltration types proposed for use in this patent, are also susceptible to frequent plugging and thus deleteriously increase the operating cost. For these reasons, the filtration process proposed in this patent has not gained widespread commercial acceptance.
Accordingly, a need exists for more efficient and economical manners of recovering oil from byproducts created during the dry milling of corn to produce ethanol.